This invention relates to resilient connectors and, in particular, to a resilient probe including a spring made from a tube having a helical cut along its length.
Resilient probes have been used for many years to make electrical connections to printed circuit boards and hybrid substrates for testing. In the past, electrical contacts on printed circuit boards were relatively widely spaced, e.g. 50-200 mils (1.3-5.0 mm) center to center. Integrated circuits have become much smaller and contact spacing has decreased, making it increasingly difficult to obtain a reliable connection for testing. For example, surface mount devices have a contact spacing of 8-15 mils (0.2-0.4 mm).
Commercially available resilient probes have four or five components, typically including an elongated probe body or barrel having a spring in a closed end and a plunger resting on the spring and extending from an open end of the barrel. A portion of the plunger fits closely within the barrel to provide electrical contact between the plunger and the barrel. The barrel can be mounted directly in a fixture in an array with other probes or can be mounted in a conductive tube or receptacle which is mounted in a fixture in an array with other probes. When the tip of the plunger is pressed onto a contact area, the plunger is pushed into the barrel, compressing the spring against the closed end of the barrel. For example, U.S. Pat. No. 5,004,977 (Kazama) discloses such a probe.
Resilient probes of the prior art rely on a wiping contact between the plunger and the barrel. The wiping contact wears after a number of operations and may degrade electrical performance. In addition, the rubbing surfaces are subject to corrosion and particle contamination. The prior art has attempted to overcome this problem by soldering one end of the spring to the plunger and soldering the other end of the spring to the barrel.
A probe in which the spring is not soldered conducts a signal from the tip of the probe through the wiping contact between the plunger and the barrel to a lead attached to the barrel. Soldering the spring may eliminate the problem of intermittent contact but introduces an inductor into the circuit: the helical spring. When a soldered spring is used, the signal is conducted from the tip of the plunger through the spring to the barrel. The inductance of the spring can be significant at very high frequency, especially for circuits generating square waves rather than sinusoidal waves. The inductance can cause degradation of the signal, e.g. distortion of a square wave, phase distortion, or impedance changes.
Resilient probes tend to be electrically long in terms of the wavelength of the signal passing through the probe. For example, a probe 7.5 cm long is one quarter wavelength long at 1000 Mhz. It is preferred to have a probe much less than one tenth of a wavelength long to minimize problems with discontinuities and reflections. The several components in a probe of the prior art can cause discontinuities and reflections to occur. A coaxial probe of the prior art is electrically like a coaxial cable having a number of joints or connections in the cable. Each joint in the cable is a discontinuity and can cause reflections, particularly if adjoining segments of the cable do not have the same impedance. The same is true within a probe.
In addition to close pin spacing and very high frequency signals, modern printed circuit boards often have concentric contact areas. For example, some devices have a lead which is not grounded but is held at ground potential. This "driven ground" is difficult or impossible to access with a probe. Another problem is the separation of voltage and current terminals for resistance measurements. Precise measurement of resistance requires that the current through a device be applied from a first set of terminals and the voltage drop across the device be measured from a separate pair of terminals. Otherwise, one is measuring the voltage drop of the device plus the voltage drops across the two contacts to the device. Since the current and voltage terminals are closely space, probes of the prior art have not been able to separately access these terminals. Thus, there is a need in the art to be able to contact concentric or closely spaced terminals, e.g. signal, driven ground, and electrical ground or current, voltage, and ground terminals.
The restoring force in a typical resilient probe comes from a compressed, helical spring. As industry attempts to miniaturize probes to keep pace with circuit density, there is a problem in that reducing the size of the spring reduces the restoring force provided by the spring. This limits the amount one can miniaturize a resilient probe constructed as known in the prior art.
A helical spring increases in diameter when compressed, requiring that the inside diameter of the barrel be larger than the nominal outside diameter of the spring in order to provide adequate clearance for the spring and to prevent binding. Binding is particularly likely when the plunger is subjected to excessive travel, possibly jamming one coil between another coil and the barrel. The variable diameter of the spring may adversely affect high frequency performance, especially in a coaxial probe.
In many applications, e.g. hard drives for computers and optical disc players, a resilient connection is made to a linearly moving head for reading or writing data. Typically, the resiliency of fine wires is relied upon for a resilient electrical connection between the head and the rest of the circuitry. For many applications, it is desired to obtain a more rugged, but no less resilient, connection. Particularly for data scanning applications, mechanical stability and repeatable motion are of paramount importance.
In view of the foregoing, it is therefore an object of the invention to provide a resilient connector which can easily be miniaturized.
Another object of the invention is to provide a resilient probe suitable for very high frequency signals.
A further object of the invention is to provide a resilient probe which can be arranged with like probes in an array having a center to center spacing of less than 10 mils.
Another object of the invention is to provide a resilient probe having fewer discontinuities than probes of the prior art.
A further object of the invention is to provide a resilient connector having a large over-drive capability.
Another object of the invention is to provide a resilient connector having improved stability and repeatable motion.